Acoustical measuring device



Patented Dec. 26, 1939 i UNITED STATES PATENT OFFICE ACOUSTICAL MEASURING DEVICE Herbert Belar, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application September 30, 1936, Serial No. 103,407

6 Claims. (Cl. 181-05) This invention relates to acoustical measurlogarithm of the input current of the device I3. ing devices such as are utilized for measuring This results from the fact that the current of the reverberation time of a room, and has for its the resistor l6 varies expo y With time principal object the provision of an improved apwhen the capacitor I1 is allowed to discharge 5 paratus and method of operation whereby the throu e resistorv '6- rate of sound decay in a room may be more read- Sound produced by the loud speaker I i is ily compared with a quantity which decays logapicked up by a microphone 22 and the resulting rithmically. audio current is supplied through amplifiers 23 The measuring device of the present invention d a deteCtOI. and an amplifier 25 to an is in some respects an improvement on the output resistor 21, which is connected with the 19 reverberation time bridge described by 01 11 and resistor H in a circuit including an indicator or Kreuzer in The Journal of the Acoustical Someasuring dev ce The loud speaker ii d ciety of America, July 1930, Vol. 11, No. 1, pages microphone 22 are located within the room which '78 to 82. This Olsen and Kreuzer devi e f is to have its acoustical characteristics measured.

15 ti n t compare t r t of Sound decay t a The current of the resistor 21, like that of re- 15 quantity which varies logarithmically. In the sister I9, varies directly as the lo a of the operation of this device it has been found that input current of the device 3. r the ratio between the compared quantities or'cur- The Operation 0f e device includes n rents is so great, especially in the lower value and Opening of the Switch When this w h 2 ranges, that it is diificult to secure an accurate is se t oscillator l0 feeds po e of the d indication of their resultant diiference. In acsired frequency to the loud speaker H. T e cordance with the present invention, this difiisound pressure generated by the loud Spe is culty is avoided by the provision of means for p fi by t amplifiers 23 and 24 in t e V producing an electrical current which varies or form f the electrical energy p t0 the m cro- 0,5 decays in accordance with the logarithm of the p The Output of the amplifier 24 s fed 25 sound pressure in a room. Stated more specifit0 the detector 25 Which Produces a direct 111- c'ally, the invention involves the use of a detecrent Output Voltage dependent 011 Sound D stor and amplifier, the output of which v ri sure. This direct current output is fed to the linearly with the logarithm of sound pressure and device w c is 50 adjusted y means of seaffords a more favorable basis of comparison sister 29 that grid current is just about to start. 30 with current which varies logarithmically and is s adjustment maybe determined y pp yconverted to similarly vary linearly. ing a signal to the input of the amplifier, meas- The invention will be better understood from uring the O p current of tube With switch the following description when considered in conl2 p and plotting the Output current st nection with the accompanying drawing, and its the logarithm of the input voltage for various 5 scope is indicated by the appended claims. adjustments of the resistor 29. When the out- The single figure of the drawing is a wiring put current shows a linear relationship to the diagram of the improved reverberation measurlogarithm of the input voltage over the widest ing device. range, the resistor 29 is properly adjusted. At

This device includes an oscillation generator this adjustment it will be found that grid cur- 40 I0 arranged to be connected to a loud speaker ll rent definitely flows when an appreciable signal through one blade of a switch l2. The other is applied, although without signal there is no blade of this switch completes the input circuit appreciable grid current.

a of a channel including an electron discharge de- The resistor I5 is adjusted in a similar manvice l3 provided with an input circuit including ner by applying a D. C. voltage to the terminals 46 series resistors 14 and I5, shunt resistor l6 and of the switch l2 which leads to the grid circuit of capacitor II. The output circuit of this device be d y again measuring the tput 0111- includes a resistor l8 and a resistor l9, the porent of the said tube and plotting it against the tential drop of which is to be used as a basis of input e- 50 measurement. Input and output voltages for By reason of a large resistor 0 in Series With 50 operating the device l3 are derived from a source the grid of the device this input Voltage D shown as a battery 20, the input potential being duces in the plate circuit of the device 26 a curapplied through the adjustable terminal of a rerent which is proportional to the logarithm of the sistor 2|. This channel operates to produce in input voltage, and since the logarithm of the inthe resistor IS a current which varies as the put voltage varies with the decay of the sound, 55

the logarithm of the input voltage and the plate current of the device 26 both vary linearly with time when the sound is stopped. At the same time, the device l3 draws current due to a positive potential being applied from the battery 20 across the resistor l5 by adjustment of the reversely connected resistors I9 and 21 and this current, due to the characteristics of the-circuit, decreases linearly with time when the oscillator output is stopped. The output of the devices I3 and 26 are balanced; When the switch I2 is opened, the sound in the room decays logarithmically, thereby causing a decrease in current through the resistor 21 which is linear with time. Likewise, when the switch I2 is opened, the voltage across the capacitor decreases logarithmically with time due to leakage through resistor l6, and due to the action of the tube l3, current through resistor I9 decreases linearly with time.

Sincethe meter 28 is connected between the plates of tubes 26 and I 3, no current flows through it when the resistors 21 and H! are so adjusted that the plate voltages of tubes 26 and 13 are the same. If one of the tubes draws more current than the other, the meter will show a deflection in a corresponding direction, but similar variations in both plate currents will not deflect the meter.

With the apparatus adjusted to equilibrium, as pointed out above, the switch I2 is opened and closed and resistor I6 is adjusted for minimum deflection, of meter 28 during the period of decay of sound in the room. The reverberation time of the room then corresponds to the time constant of the circuit consisting of capacitor l1 and resistor l6, and may be mathematically determined from their values.

I claim as my invention:

1. An acoustical measuring device including a detector provided with an output circuit, means in said circuit for producing a current which varies linearly with the logarithm of sound pressure, means for producing a current which varies linearly with time, and means operatively connected to said last means .and said circuit for balancing said currents one against the other.

2. An acoustical measuring device including a detector provided with an output circuit, means comprising a resistor connectedin said circuit for producing a current which varies linearly with the logarithm of sound pressure, means for producing a current which varies linearly with time, and means operatively connected to said lastmeans and said circuit for balancing said currents one against the other.-

3. An acoustical measuring device including a detector provided with an output circuit, means in said circuit for producing a current which varies linearly with the logarithm of sound pressure, means for producing a current which varies linearly with time, means for balancing said currents one against the other, and means operatively connected to said last means and said circuit for indicating the resultant of said currents.

4. An acoustical measuring instrument including a plurality of channels each provided with an input and an output circuit, means in one of said input circuits for introducing into said input circuit a current which varies linearly with sound pressure, means in another of said input circuits for introducing into said other input circuit a current which varies logarithmically with time, means for producing in said output circuits currents which vary directly as the logarithm of their corresponding input currents, and means connecting said output circuits for indicating the resultant difierence of said output currents.

5. An acoustical measuring instrument including a plurality of channels each provided with an input and an output circuit, 'means in one of said input circuits for introducing into said input circuit a current which varies linearly with sound pressure, means in another of said input circuits for introducing into said other input circuit a current which varies logarithmically with time, means for producing in said output circuits currents which vary exponentially with their corresponding input currents, and means connecting said output circuits for indicating the resultant difference of said output currents.

6. An acoustical measuring device including a detector provided with an output circuit, means .in said circuit for producing a current which varies linearly with the logarithm of sound pressure and which, therefore, varies linearly with time on the cessation of sound input, means for producing a current which varies linearly with time on the cessation of an input independent of sound, means for simultaneously cutting ofi both of said inputs, means operatively connected to said current producing means and said circuit for balancing said currents one against the other,

and means for indicating the resultant of said 

